These words of Victor Frankenstein are from Chapter 5; however, Victor's anxieties are already suggested in Chapter 4 as he tells Walton how engrossed he had become in his work. While he has been enthusiastic about his work, at the same time, Victor has some misgivings about what he has been doing:
Every night I was oppressed by a slow fever, and I became nervous to a most painful degree; the fall of a leaf staratled me, and I shunned my fellow-creatures as if I had been guilty of a crime.
Having "lost all soul or sensation but for this one pursuit," Victor has lost much of his essence as a person and become obsessed with only the creation of a man. So, on the November night when he tries to imbue life into the "lifeless thing" that lies at his feet, he feels "an anxiety that almost amounted to agony." Wondering what will occur, in what form his long efforts will result, causes an angst within Victor Frankenstein. He agonizes that he may have exceeded the reach of a human being; he is tortured by this reach that may not become what he has desired.